The aim of this proposal is to identify the scope and limitations of bridgehead carbocations in organic synthesis. The synthesis of polycyclic natural products containing bicyclo[2.2.2]octane, bicyclo[3.2.1]octane and bicyclo[3.3.1]nonane subunits will be accomplished. Biologically significant molecules such as aphidicolin (anticancer agent, antiviral agent) will be synthesized. Since bridgehead carbocations have not been previously employed in synthesis, this proposal should make available new and efficient synthetic pathways.